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Old 12-08-2017, 08:04 PM
Crist Rigott Crist Rigott is offline
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Location: Waxahachie, TX
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BK 415 Repair

A generous member of this Forum gave me a BK 415 and a 1077B abd most all the cables. A big thank you to you know who you are.

When I got home and finished the Zenith 19" Porthole chassis I did the first IF alignment using my Knight KG-687 sweep generator. Then I remembered the 415!

I cleaned it up and changed the mic connectors for BNC and made up the missing cable. All very easy. I remembered that Bob Andersen had a video on using one of these. After searching his videos I found where he got one and turned it on for the first time and did a preliminary check of his 415. so I did the same thing. Uh, oh. mine wasn't working very well at all. Looks like no 44mc sweep!

I DL the manual and printed out the pages I thought I'd need. The first picture shows what to check when there is no IF sweep. Cool.

So out came the MM and I did some voltage checks per this page:

and this page:

I also checked the 10.7mc section of the same board just as a comparison. Looks like the 44mc sweep transistor Q-201 doesn't match the voltage chart. Might need a new transistor. It is marked as a MRF 502. I checked the other components near it and didn't see anything out of order.

Then I noticed the manual has a list of a bunch of voltage "TP" spots. I then check out each one. Then I noticed a resistor that looked like it was burnt. Not a char burn but a light color dusting on it. It is R56 on the Waveform Generator. It's a 100 Ohm 1/2w resistor. I replaced it with a 100 Ohm 1W resistor. I turned the 415 back on. In a few seconds it got hot also. Hmm...something must be shorted. I looked on the schematic and saw that a tantalum capacitor C52 goes between the 100 Ohm resistor and ground. I lifted the ground lead and turn the 415 back on. R56 didn't get hot! I didn't have a 10uf 35V tantalum but I did have a 6.8uf 25V one and used that for now. I've ordered some caps and a few MRF 502 transistors.

The manual has a page that showed some wave forms and what their voltages should be. I then used my scope to check them. All voltages looked reasonable with the exception of the base voltage on both Q-12 and Q-13. The voltage measured was about 1/2 of what they should be. 2V vs 1V and 1V vs .5V. I haven't had a chance to look into it any further.

Here is a picture of the 44mc and 10.7 sweep board and its schematic just for reference.

Last edited by Crist Rigott; 12-08-2017 at 09:01 PM.
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Old 12-08-2017, 08:47 PM
Crist Rigott Crist Rigott is offline
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I noticed these caps across the line input. Should I replace these with safety caps?

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Old 12-09-2017, 11:39 AM
kf4rca kf4rca is offline
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Those caps are probably fine. I'd check all the high value resistors like 100k, 220k, 270k, 330k, 470k, 820k, 1M, 2.2M, etc. High value carbon resistors of that vintage are famous for changing value.
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